When/how does this show up? This could be an ad or virus. Go to the [https://addons.mozilla.org/en-US/firefox/ Mozilla Add-ons Web Page] {web Link} (There’s a lot of good stuff here) and search for a good ad blocker. ----------- You may have ad/mal-ware. Further information can be found in this article; https://support.mozilla.org/en-US/kb/troubleshoot-firefox-issues-caused-malware?cache=no Run most or all of the listed free to use malware scanners. Each works differently. If one program misses something, another may pick it up.